Don't your mount your device via USB and copy files to the card? This means it is writeable inside the device.

That could theoretically be just a function of the USB bridge chip. More to the point, though, the Most Recently Read folder in the root of the SD card is in fact a file that the V3 updates when a book is read. The same is true for the bookmarks, which are stored as one file per book in another folder in the root of the SD card